The present invention relates to reactors for chemical reduction of nitrogen oxide (NOx) emissions in the exhaust gases of automotive engines, particularly diesel and other engines operating with lean air fuel mixtures that produce relatively high emission of NOx and method of manufacturing the same. More particularly, the invention relates to a low-loss electrode-printed structural dielectric barrier for a non-thermal plasma reactor and to improved non-thermal plasma reactor multi-cell stacks having structural dielectric barriers printed with low-loss electrode patterns.
In recent years, non-thermal plasma generated in a packed bed reactor has been shown to be effective in reducing oxides of nitrogen (NOx) produced by power plants and standby generators. These units usually have a reducing agent, such as urea, to enhance the conversion efficiency. The packed bed reactor consists essentially of a high voltage center electrode inserted into a cylinder of dielectric material, usually a form of glass or quartz. An outside or ground electrode is formed by a coating of metal in various forms, including, tape, flame spray, mesh, etc. The space between the center electrode and the inside diameter of the dielectric tube is filled or packed with small diameter glass beads. When high voltage alternating current is applied to the center electrode, the surfaces of the beads go into corona, producing a highly reactive and selective surface for inducing the desired reaction in the gas.
Unfortunately, the packed bed design with loose beads and glass dielectric is impractical for use in the conditions found in a mobile emitter, such as a car or truck. The vibration and wide temperature swings of the vehicle system would damage the packed bed and the necessary temperature and vibration isolation needed to make it survive would not be cost effective.
A non-thermal plasma reactor for use with diesel engines and other engines operating with lean air fuel mixtures is disclosed in commonly assigned U.S. patent application Ser. No. 09/465,073, filed Dec. 16, 1999, entitled xe2x80x9cNon-thermal Plasma Exhaust NOx Reactor, which is hereby incorporated by reference herein in its entirety.xe2x80x9d Disclosed therein is a reactor element comprising high dielectric, nonporous, high temperature insulating means defining a group of relatively thin stacked cells forming gas passages and separated by the insulating means. Alternate ground and charge carrying electrodes in the insulating means on opposite sides of the cells are disposed close to, but electrically insulated from, the cells by the insulating means. The electrodes may be silver or platinum material coated onto alumina plates. Conductive ink is sandwiched between two thin nonporous alumina plates or other suitable insulating plates to prevent arcing while providing a stable electrode spacing for a uniform electric field. The electrodes are coated onto alumina in a pattern that establishes a separation between the electrodes and the connectors of alternate electrodes suitable to prevent voltage leakage.
U.S. Pat. No. 6,338,827 to Nelson et al., commonly assigned, entitled xe2x80x9cStacked Shape Plasma Reactor Design for Treating Auto Emissions,xe2x80x9d which is hereby incorporated by reference herein in its entirety, discloses a non-thermal plasma reactor element prepared from a planar arrangement of formed shapes of dielectric material. The shapes are used as building blocks for forming the region of the reactor wherein the plasma is generated. Individual cells are provided with a conductive print disposed on a formed shape to form electrodes and connectors. In a preferred embodiment, the conductive print comprises a continuous grid pattern having a cutout region disposed opposite the terminal connector for reducing potential charge leakage. Multiple cells are stacked and connected together to form a multi-cell reactor element.
Commonly assigned U.S. patent application Ser. No. 09/517,681, filed Mar. 2, 2000 entitled xe2x80x9cPlasma Reactor Design for Treating Auto Emissionsxe2x80x94Durable and Low Cost,xe2x80x9d which is hereby incorporated by reference herein in its entirety, discloses a non-thermal plasma reactor element for conversion of exhaust gas constituents. The reactor comprises an element prepared from an extruded monolith of dense dielectric material having a plurality of channels separated by substantially planar dielectric barriers. Conductive material printed onto selected channels forms conductive channels that are connected along bus paths to form an alternating sequence of polarity, separated by exhaust channels. Conductive channels and channels not selected for exhaust flow are plugged at end portions of the monolith with a material suitable for excluding exhaust gases and preventing electrical charge leakage between conductive channels. Exhaust channels, disposed between opposite polarity conductive channels, are left uncoated and unplugged. During operation, exhaust gas flows through channels and is treated by the high voltage alternating current plasma field. The planar shape of the dielectric barriers provides a uniform electrical response throughout the exhaust channels.
U.S. Pat. No. 6,354,903 to Nelson et al., commonly assigned, entitled xe2x80x9cMethod of Manufacture of a Plasma Reactor with Curved Shape for Treating Auto Emissions,xe2x80x9d which is hereby incorporated by reference herein in its entirety, discloses a non-thermal plasma reactor element wherein a swept shape substrate is formed and treated to create a non-thermal plasma reactor element. The substrate is formed via extrusion so that there is a series of nested, concentric dielectric barriers. Selected channels are coated with conductive material to form conductor channels capable of forming an electric field around exhaust channels. Conductive channels and channels not selected for exhaust flow are plugged at end portions of the monolith with a material suitable for excluding exhaust gases and preventing electrical charge leakage between conductive channels. Exhaust channels, disposed between opposite polarity conductive channels, are left uncoated and unplugged.
U.S. Provisional Application No. 60/249,231, of David E. Nelson, et al., filed Nov. 16, 2000, entitled xe2x80x9cEdge-connected Non-thermal Plasma Exhaust After Treatment Device,xe2x80x9d which is hereby incorporated by reference herein in its entirety, discloses an edge-connected non-thermal plasma reactor substrate including an edge-connected frame comprising a pair of dielectric edge connectors secured at opposite ends to first and second outer dielectric plates. The dielectric edge connectors comprise a backplane and a plurality of tines protruding along at least one major surface of the backplane, the plurality of tines being spaced apart from one another at regular intervals so as to form pockets between adjacent tines. A plurality of alternating polarity electrode plates are disposed within the edge-connected frame in an alternating polarity arrangement that defines the presence of at least one dielectric barrier next to a plasma cell with the pockets compliantly engaging opposite ends of the electrode plates.
While the above-described non-thermal plasma reactors meet some of the current needs and objectives in the art, there remain several issues that need to be more effectively addressed.
Current stacked planar designs without structural ligaments are prone to fracture during use in automotive exhaust applications if the unsupported span of the dielectric barrier becomes large (fracture is often observed when the length of unsupported span approaches about 30 millimeters for a 0.5 millimeter thick alumina dielectric barrier). This failure can be induced by gas flow and vibration induced stresses leading to deflection of the dielectric barriers. NOx conversion efficiency can be negatively affected even with slight deflection of the dielectric barriers due to gas by-pass between plates or from plasma field variation.
Current non-thermal plasma reactor elements having structural ligaments provide improved durability over ligament-free elements. However, the improved durability comes at a cost of lower NOx conversion efficiency than is achieved over a similar length region with ligament-free designs. While structural ligaments are desirable for increasing structural durability, these same structural ligaments have a deleterious effect on non-thermal plasma reactor conversion efficiency due to their interaction with the electric field in the plasma.
Many currently known stacked planar reactor elements are expensive to manufacture. Planar designs using metallized dielectric plates and discrete spacers need fixturing to hold each spacer in place relative to the metallized dielectric plates during assembly, and there are many discrete parts that must be handled. Formed C-shapes or box shapes simplify the stack assembly process since relatively simple tooling is used to align the stack. However, as a result of these and other issues, there remains a need in the art for a non-thermal plasma reactor element providing improved reactor durability and improved NOx conversion efficiency at reduced cost.
In a first embodiment, the present invention provides a multi-cell non-thermal plasma reactor stack comprising:
a plurality of alternating polarity unit cells, individual unit cells comprising a pair of low-loss electrode-printed structural dielectric barriers;
the electrode-printed structural dielectric barriers having a first side and a second opposite side;
the second opposite side comprising a flat surface having a low-loss electrode pattern; the low-loss electrode pattern comprising first and second major electrode sections that are offset from the plan view of ribs, ligaments, spacers, tines, or other dielectric support structure present between dielectric barriers in a multi-cell reactor, a connector disposed between and electrically connecting the first and second major electrode sections and offset relative to a centerline extending through the first and second major electrode sections, and a bus path connector electrically connected to one of the first or second major electrode sections and to the side bus path and offset relative to the centerline;
at least one of the low-loss electrode-printed structural dielectric barriers in each unit cell pair comprising a ribbed structural dielectric barrier;
the ribbed structural dielectric barrier comprising a pair of side structural ribs disposed at first and second ends of the first side of the low-loss electrode printed structural dielectric barrier and at least one internal structural rib disposed between the first and second side structural ribs;
the side structural ribs and internal structural rib being an integral part of the structural dielectric barrier; and
exhaust channels provided between the pair of low-loss electrode-printed structural dielectric barriers forming each unit cell, the exhaust channels being defined by the side structural ribs.
In a second embodiment, the present invention provides a low-loss electrode-printed structural dielectric barrier for a non-thermal plasma reactor comprising:
a structural dielectric barrier having a first side and a second opposite side;
a low-loss electrode pattern disposed on the second opposite side of the structural dielectric barrier;
the low-loss electrode pattern comprising first and second major electrode sections that are offset from any ribs, supports, ligaments, spacers, tines, or other structure that serves as a structural dielectric connection between dielectric barriers in a multi-cell reactor; a connector disposed between and electrically connecting the first and second major electrode sections and offset relative to a centerline extending through the first and second major electrode sections; and a bus path connector electrically connected to one of the first or second major electrode sections and to a side bus path and offset relative to the centerline.
In a third embodiment, the present invention provides a method for preparing a multi-cell non-thermal plasma reactor stack comprising:
preparing a plurality of low-loss electrode-printed structural dielectric barriers, wherein the low-loss electrode-printed structural dielectric barriers comprise ribbed electrode-printed structural dielectric barriers or non-ribbed low-loss electrode printed structural dielectric barriers, comprising:
forming a structural dielectric barrier having a first side and a second opposite side;
firing the structural dielectric barrier;
printing a low-loss electrode pattern onto the second opposite side of the fired planar structural dielectric barrier;
the low-loss electrode pattern comprising first and second major electrode sections that are offset from the plan view of ribs; a connector disposed between and electrically connecting major electrode sections and offset relative to a centerline extending through said first and second electrode sections perpendicular to the rib orientation; and a bus path connector electrically connected to one of the major electrode areas and to the side bus path and offset relative to the centerline;
to provide a non-ribbed low-loss electrode printed structural dielectric barrier; or
optionally, forming on the first side of said structural dielectric barrier a pair of side structural ribs disposed at first and second ends of the dielectric barrier and at least one internal structural rib disposed between the pair of side structural ribs to provide a ribbed low-loss electrode printed structural dielectric barrier;
stacking a plurality of alternating polarity unit cells to form a multi-cell stack;
the unit cells having exhaust channels provided between pairs of low-loss electrode-printed structural dielectric barriers, the exhaust channels being defined by the side structural ribs;
wherein individual unit cells in the multi-cell stack comprise a pair of ribbed low-loss electrode-printed structural dielectric barriers; the ribbed low-loss electrode printed structural dielectric barriers having at least one internal structural rib and a pair of side structural ribs disposed at first and second ends of said electrode-printed planar dielectric barrier; or
wherein individual unit cells in said multi-cell stack comprise a non-ribbed low-loss electrode-printed structural dielectric barrier and a ribbed electrode-printed structural dielectric barrier.
In a fourth embodiment, the present invention provides a method for preparing low-loss electrode-printed structural dielectric barriers for non-thermal plasma reactors comprising:
forming a structural dielectric barrier having a first side and a second opposite side;
firing the structural dielectric barrier;
printing a low-loss electrode pattern onto the second opposite side of said structural dielectric barrier;
the low-loss electrode pattern comprising first and second major electrode areas that are offset from the plan view of ribs, supports, ligaments, spacers, tines, or other structure that serves as a structural dielectric connection between dielectric barriers in a multi-cell reactor, a connector disposed between and electrically connecting the first and second major electrode sections and offset relative to a centerline extending through said first and second major electrode sections, and a bus path connector electrically connected to one of the first and second major electrode areas for connecting to a side bus path and offset relative to the centerline.
The present low-loss electrode non-thermal plasma reactor multi-cell stacks advantageously employ structural ribs formed as an integral part of a structural dielectric barrier (such as the embodiment comprising an E-shaped ceramic structural dielectric barrier). The flat, non-ribbed side of the structural dielectric barrier is printed with the present low-loss electrode pattern that is especially advantageous for minimizing parasitic losses encountered in previously available reactors having structural support ribs. The present low-loss electrode pattern is segmented to provide first and second major electrode sections, and each electrode section is patterned so that there is an offset distance between the structural ribs and the electrode section that is approximately equal to the distance between the pair of low-loss electric printed dielectric barriers forming the unit cell (i.e., approximately equal to the height of the exhaust channel). For ease of discussion, the low-loss electrode is discussed with respect to the offset between the electrode sections and ribs. Ribs, within the scope of the present invention, may alternately be ligaments, spacers, tines, or other dielectric support structure that serves as a structural dielectric connection between dielectric barriers in a reactor. The offset distance (and exhaust channel height) is typically about 1 to about 2 millimeters. This offset distance advantageously reduces parasitic losses by optimizing the effect of the plasma field. At the edge of the electrode pattern, a fringing charge field forms during reactor operation and the present offset allows the fringing field to treat gas flowing through the offset region while substantially limiting the electric field that is exposed to the structural rib (or other dielectric support structure). Since the structural rib is not substantially exposed to the electric field, any deleterious effect the rib may have on the plasma field is reduced or eliminated altogether thereby enhancing NOx conversion efficiency. The actual offset distance between the segmented electrode pattern and the structural ribs is fine-tuned by empirical testing.
The segmented electrodes have thin connecting paths that extend over one side structural rib (i.e., a connecting path providing the bus path connector to power or ground) and any internal ribs (i.e., a connecting path providing electrical connection between electrode sections). The connecting path width is selected to provide a width sufficient to accommodate the maximum current flow. The connecting paths are offset relative to a centerline extending through the electrode so that when assembled into a multi-cell reactor, there is increased distance between opposite polarity connection paths within and around the dielectric ribs, ligaments, spacers, tines, or other dielectric support structure. The increased distance reduces the effective charge acting on dielectric ribs to minimize parasitic losses.
The low-loss electrode pattern of the present invention is advantageously employed in any parallel gap non-thermal plasma reactor. For example, the present low-loss electrode pattern is advantageously employed in multi-cell reactor stacks using discrete spacers and edged-connected planar non-thermal plasma reactors designed to minimize spacer or ligament-related parasitic losses as taught in commonly assigned U.S. Provisional Application No. 60/249,231, of David E. Nelson, et al., filed Nov. 16, 2000, entitled xe2x80x9cEdge-connected Non-thermal Plasma Exhaust After Treatment Device.xe2x80x9d
The present low-loss electrode printed structural dielectric barrier non-thermal plasma reactor multi-cell stack provides improved durability and NOx conversion efficiency along with reduced manufacturing costs over previously known reactors. Advantageously, the low-loss electrode pattern can be used for opposite polarity dielectric plates in the multi-cell arrangement thereby simplifying manufacturing.
These and other features and advantages of the invention will be more fully understood from the following description of certain specific embodiments of the invention taken together with the accompanying drawings.